Improvement Of The UV Electroluminescence Of ZnO/MEH-PPV Heterojunction Devices And Study Of Luminescence Mechanism

As a wide direct band gap semiconductor, Zinc oxide is drawing more attention as the need of ultraviolet (UV) light emitting diodes rises. Light emitting diodes using ZnO nanostructures becomes a research hotspot. However, the p-type doping ZnO is always a bottleneck since wide band gap materials usually show a poor doping efficiency. We have reported a device based on a ZnO nanorods/MEH-PPV heterosturcture in which the ZnO nanorods were fabricated by hydrothermal growth method and the polymer MEH-PPV was used as the electron injection layer. Under DC bias, the near ultraviolet (N-UV) electroluminescence of ZnO at380nm was detected. But there are several problems in the work:The spectrum is not pure, the turn-on voltage of the device is high, the electroluminescence (EL) power is weak, the carrier injection and transport are unbalance, and the luminescence mechanism is not clear and so on. This paper makes the following research work to improve ZnO Nanorods/MEH-PPV heterostructure devices and solve above-mentioned problems.Firstly, Al-doped ZnO(AZO) as a new kind of electrode is firstly used to replace ITO in our experiments. The device of AZO/ZnO/ZnO nanorods/MEH-PPV/Al has been fabricated. Under DC bias, UV EL at380nm from ZnO band edge emission is observed. Because of the higher work function and lattice matching, AZO is more suitable as the electrode of ZnO Nanorods/MEH-PPV heterostructure devices than ITO. The turn-on voltage of the device with AZO as the electrode is lower than that of ITO, and the EL power is enhanced. When a0.8nm thick LiF film is deposited on the MEH-PPV layers by thermal evaporation, the turn-on voltage of the device is further decreased and the EL power is obviously increased.Secondly, the polymer PMMA is considered to be able to solve the impurity of the spectrum of ZnO Nanorods/MEH-PPV heterostructure devices by contrast analysis of PMMA and MEH-PPV. The reasonable explanation is stated about using PMMA as the organic layer, which is the electrons tunneling theory. As ZnO nanoparticle films/PMMA heterostructure devices studied by us cannot utilize field emission of ZnO nanorods points, ZnO Nanorods/PMMA heterostructure device has been fabricated. The results show that the turn-on voltage of the device with ZnO Nanorods is lower than that of ZnO nanoparticle films, and the EL power is stronger. But the stability of ZnO Nanorods/PMMA heterostructure device needs to be improved. The reason why UV EL power is enhanced is that field emission of ZnO nanorods points increases the probability of electrons tunneling.Finally, the using of n-ZnO and cathode modification is discussed. ZnO nanorods are fabicated directly on organic material MEH-PPV by hydro thermal method. Only ZnO defect luminescence is observed from the heterostructure device, which means that low temperature annealing cannot get ZnO nanorods with good cryatallinity. The reason why N-UV electroluminescence cannot be detected from anti-configuration device using Au as cathode is that the voltage sharing of organic and inorganic layer and carrier transporting is unbalanced. Adding PFN as cathode modification layer can promote electrons injection, enhance the EL from MEH-PPV and restrain the UV EL from ZnO, which means that UV EL at380nm from ZnO is mainly based on electrons tunneling.